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Monday, November 15, 2010

Last week I needed to simulate an chirped RF pulse signal for some testing I was doing. Chirped pulse signals are often used in radar and electronic warfare applications as a pulse compression technique. Not having created a signal like this before I sent a quick email to an RF/Microwave AE colleague of mine asking how they did it. They sent me a quick reply back saying I should use Agilent's VSA software and a vector signal generator. The problem is I did not have a license for the VSA software and I did not have a vector signal generator (vector sig gens can easily be double the price of an analog). With limited time to get the test done I had to figure out a way to do it with my N5183A MXG analog signal generator. Now a chirped RF pulse is a pulse modulated RF carrier, but the carrier is swept (for more info check out this Wikipedia page Chirp). My MXG did have pulse modulation and FM (to sweep the carrier) built-in. I used these functions to create a chirped RF pulse signal. The problem with my chirped RF pulse signal was there was no easy way to sync the pulse modulation and FM so from pulse to pulse the sweep was always starting at a different value. To fix that problem I used the 33522A 2-channel function / arbitrary generator that has the ability to sync its outputs, meaning I could control the phase relationship between the channel 1 waveform and the channel 2 waveform. I set the MXG's pulse modulation source and FM source for external. Channel 1 of the 33522A served as the pulse modulation source. I wanted to linearly sweep the carrier so I set channel 2 of the 33522A, my FM source, for a linear ramp waveform. You can see my setup in the image below:

A positive sloped ramp gives you a low to high frequency sweep and a negative sloped ramp gives you high to low frequency sweep. The sweep speed of the chirp is set by the rate of the slope. The frequency range of the sweep is set by the FM deviation setting on the MXG. You can see the function generator outputs on the scope screen in the image. The start of the pulse modulation on the MXG (bottom left) is controlled by the rising edge of the pulse waveform from channel 1 of the 33522A. If you notice in the image, the ramp waveform is already approximately halfway through its ramp up cycle by the time the pulse is triggered. This was done intentionally in fact you can control where the carrier sweep starts and stops inside the pulse by adjust the phase of the ramp in relation to the pulse output. On the bottom right, the N9020A MXA signal analyzer shows the resulting chirped RF pulse in the frequency domain. Below the 33522A in the image, the 53230A universal counter's microwave channel is also being used to measure the pulse characteristics including: pulse width, pulse repetition rate, and carrier frequency.
The analog signal generator and 2-channel function generator method for creating chirped RF pulse signals filled my testing needs and was much lower cost then turning to a vector signal generator.